Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
2000-03-03
2004-05-04
Pascal, Leslie (Department: 2633)
Optical communications
Multiplex
Wavelength division or frequency division
C398S181000, C398S192000, C398S193000, C398S199000
Reexamination Certificate
active
06731877
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to method and apparatus for transmitting optical signals in high-capacity, long distance, lightwave communication systems which use Return-To-Zero (RZ) modulation format and dispersion management.
BACKGROUND OF THE INVENTION
Optical nonlinearities of optical transmission fibers have become limiting factors for long distance, high capacity lightwave transmission systems. In an optically amplified transmission system transmission system, the amplified spontaneous emission (ASE) noise degrades the signal-to-noise ratio (SNR), and a higher signal power is then required to maintain a minimum SNR. However, optical nonlinearities distort the transmission signal and thus limits the maximum optical power that can be launched over the optical transmission line.
It is possible to balance the self-phase-modulation (SPM) with chromatic dispersion by a proper design of the transmission pulse waveform as well as the pulse energy. Such nonlinear pulses are known as “optical solitons”. Since the chromatic dispersion is compensated for by optical nonlinearity, there is no need to perform dispersion compensation in soliton systems. In this regard see, for example, U.S. Pat. No. 4,558,921 (A. Hasegawa), issued on Dec. 17, 1985, U.S. Pat. No. 4,700,339 (J. P. Gordon), issued on Oct. 13, 1987, and U.S. Pat. No. 5,035,481 (L. F. Mollenauer) issued on Jul. 30, 1991.
Although transoceanic soliton transmission is known, conventional soliton transmission technology has not been commercialized. One of the major problems with conventional soliton transmission is timing jitter. A soliton pulse width is typically approximately 10% of a bit period and there is no fundamental mechanism that can fix such short pulses in time. Perturbations such as soliton—soliton interaction, frequency shift due to the ASE noise, and an acoustic wave generated by a previous pulse tend to move the pulses out of their original position as is indicated in U.S. Pat. No. 5,710,649 (L. F. Mollenauer), issued on Jan. 20, 1998. Still further, many techniques have been used to reduce or eliminate timing jitter as is described in the book entitled “Optical Fiber Telecommunications IIIA” by I. P. Kaminow et al., at pages 373-461 (Academic Press, 1997). All the above techniques, also known as “soliton control technologies” are typically not cost effective in many practical applications, and also complicate system designs.
Recently, a new class of solitons (dispersion managed solitons) were described in the article entitled “Reduction of Gordon-Haus timing jitter by periodic dispersion compensation in Soliton transmission” by M. Suzuki et al., Electronic Letters, Vol. 31, No. 23, pages 2027-2029, 1995. An article entitled “Soliton Transmission Using Periodic Dispersion Compensation”, by N. J. Smith et al. in Journal of Lightwave Technology, Vol. 15, No. 10, pages 1808-1822, 1997, discusses a dispersion managed soliton (DMS) which has been shown to have a much better performance than a conventional soliton, while at the same time having inherent desired properties of conventional solitons in dealing with optical nonlinearities. There are five major improvements in DMS transmission systems compared to conventional soliton systems. These five improvements are (a) energy enhancement wherein it is possible to launch much higher signal power for a DMS signal than conventional soliton signals, which improves the system SNR; (b) reduced timing jitter; (c) no additional soliton control technologies are required; (d) it is compatible to existing non-return-to-zero (NRZ); and (e) it has high power and dispersion tolerance.
In an article entitled “1 Tbit/s (100×10.7 Gbits) Transoceanic Transmission Using 30 nm-Wide Broadband Optical Repeaters with Aeff-Enlarged Positive Dispersion Slope Fiber and Slope-Compensating DCF” by T. Tsuritani et al., at pages 38-39 of Post-Deadline Papers, 25
th European Conference on Optical Communications
, 1999, discloses that significant system performance has been achieved using DMS technology in many laboratory experiments in terms of both distance and total capacity. Similar performance has also been demonstrated by many other such as, for example, the articles entitled “1 Terabit/s WDM Transmission over 10,000 km” by T. Naito et al., PD2-1, pages 24-25, 25
th European Conference on Optical Communication
, 1999, and “1.1-Tb/s (55×20-GB/s) DENSE WDM SOLITON TRANSMISSION OVER 3,020-km WIDELY-DISPERSON-MANAGED TRANSMISSION LINE EMPLOYING 1.55/1.58 um HYBRID REPEATERS”, by K. Fukuchi et al., PD2-10, pages 42-43, 25
th European Conference on Optical Communication
, 1999.
Although previous laboratory experiments have proven the feasibility of DMS systems for long distance and high capacity applications, there are many challenges to achieve both reliability and flexibility such that such DMS system can be used in a realistic environment. For example, in terrestrial optical fiber networks, the distance between repeaters or optical amplifiers can be as long as 130 km. Such distances are significantly longer than those of the previous experiments. The impacts of the longer repeater spacing are two-fold. First, the required signal power is typically much higher to overcome the SNR degradation caused by a required larger amplifier gain. Second, as a result of the required higher signal power, optical nonlinearities become more important for long distance transmission. Another challenge is that there are a variety of different optical fiber types in existing optical fiber networks. Optical nonlinearities become even more detrimental for a transmission line comprising different types of optical fibers. As for DMS systems, the total capacity is limited not only by the optical amplifier bandwidth, but also the higher order chromatic dispersion of the transmission fiber. The situation is even more difficult when there are several different types of optical fibers involved. A reliable and cost-effective solution to higher order chromatic dispersion is one of the major challenges for high capacity long haul DMS systems. Finally, distance and capacity are not the only requirements for next generation optical networks. For example, it is highly desirable to have the flexibility to place optical add/drop nodes anywhere along an optical transmission line.
In a dispersion managed soliton (DMS) system, DMS predicts significant power enhancement which is valid for single channel propagation. The power enhancement cannot be fully utilized for multi-channel system since the cross-phase modulation (XPM) dominates the overall system performance. DMS systems further require an accurate balance between the self-phase modulation (SPM) in the transmission fiber and the SPM a dispersion-compensating fiber which often results in a much smaller system margin.
It is desirable to provide a high capacity ultra-long haul dispersion and nonlinearity managed lightwave communication system which overcomes the problems described hereinabove.
SUMMARY OF THE INVENTION
The present invention is directed to method and apparatus for transmitting optical signals in high-capacity, long distance, lightwave communication systems which use Return-To-Zero (RZ) modulation format and dispersion management.
Viewed from an apparatus aspect, the present invention is directed to an optical transmission system. The optical transmission system comprises a transmitter terminal and an optical transmission line. The transmitter terminal comprises a plurality of return-to-zero (RZ) transmitters and a multiplexing arrangement. Each of the plurality of return-to-zero (RZ) transmitters is adapted to receive a separate channel input signal and to generate therefrom a corresponding forward error correction (FEC) modulated output signal in a separate predetermined channel frequency sub-band of an overall frequency band which includes a predetermined channel separation from an adjacent channel frequency band generated by another RZ transmitter. The multiplexing arrangement multiplexes the plurality of the predetermined channel
Hunton & Williams LLP
Pascal Leslie
Qtera Corporation
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